Prior art has already described intervertebral cages, for example Stryker cages. These cages are parallelepiped-shaped, the bottom and the top of the cage being completely open, the side and top openings will be placed facing the two vertebrae that are to be kept at a distance from each other. Spongy bone is then compacted inside the cage to finally cause fusion of the bone (or arthrodesis) of the two vertebrae separated by a suitable disk space.
For posterior placement, there are usually two cages adjacent to each other at a distance from each other in the intervertebral space, and the graft is firstly compacted inside the two cages and secondly the space between the two cages is filled in by spongy bone or bone substitute.
It is found that the growths of the three independent grafts (two inside the cages and one between the cages) vary with respect to each other. In particular, the graft placed between the cages contributes more quickly to the fusion than the compacted grafts inside the cages. Obviously, this is a disadvantage, particularly with regard to the stability of the grafts and the two vertebrae held in place by these grafts, and concerning the time necessary to obtain good stability.
Document WO/98/55052 also discloses intervertebral cages.
In the first case, they are composed of a main very thick curved part particularly with a thickness that increases from the middle towards the ends. Therefore, these cages are complicated to make due to their curved shape.
In the second case, they are composed of a straight main part and return parts each being prolonged by an end part that together form a small interval less than 25% of the longitudinal extension of the internal housing of the cage.